E-Mobility Engineering 015 l EMotive Scarab off-road truck dossier l In Conversation: Giulio Ornella l Hall effect and magnetic sensors focus l Challenge of batteries for heavy-duty EVs l Alpha Motor Corporation digest l Automated charging insight l HVAC systems focus

The use of battery chillers has brought an additional challenge to the refrigerant loop in closely integrated systems, as they have to be powerful and very efficient to overcome the thermal resistance of a large battery pack, our engineering simulation expert says. Chiller and HVAC efficiency looks set to be improved by the return of CO 2 as a refrigerant. Known as R-744, CO 2 was in use more than a century ago before being replaced by chlorofluorocarbon and, later, HFC refrigerants that worked at lower pressures and were therefore easier on the machinery – as well as being less destructive of the ozone layer, in the case of HFCs. Although an effort by Mercedes to switch to R-744 stalled, Volkswagen has gone ahead, he reports, and others are likely to follow. Even though CO 2 is regarded as the main culprit in climate breakdown, it has a global warming potential of 1 (because it is the standard for comparison), which is much lower than that of R-134A (tetrafluoroethane) at 1430 and even of R-134A’s replacement, R1234 YF (2,3,3,3 tetrafluoropropene) at 4. As a refrigerant, R-744 is attractive because of the low temperatures the systems using it can achieve. While R-1234 YF gets down to -20 C, CO 2 systems can efficiently achieve -30 C. The price paid for this benefit is system operating pressures at around 120 bar, putting higher stresses on key components and connections. That is likely to mean larger, heavier, more power- hungry and pricier components. It will also be more difficult to keep compressors quiet, adding to the NVH challenge. As the compressor will have to meet both cooling and heating demands, and because it will run even when the vehicle is stationary and is one of the few noise sources in an EV, its NVH characteristics and its COP have become critical attributes, the HVAC systems builder says. He also points out that the added EV operational modes with greater cooling demands require more powerful electric motors to drive higher displacement compressor mechanisms, while trying to maintain or improve overall efficiency. The simulation specialist adds that more powerful, faster-spinning electric motors are likely to need liquid cooling themselves (perhaps using oil), while compressors may need redesigning to make the most of the wider speed ranges the more varied cooling/heating needs demand and the motors can deliver. Ideally, they should operate efficiently at both low and high refrigerant flow rates. Overall efficiency is the most important goal in optimising compressors and their motors for EV use, says the control hardware developer. As the compressor is the largest power consumer in the system, any improvement in efficiency translates into increased vehicle range, he adds. Raise the voltage One important way of making HVAC compressor systems more efficient is to power them with high-voltage motors. Compressors typically absorb 4-6 kW, which is more than the 12 V network can deliver without generating excessive currents, the automotive electrification specialists say. Running the compressor motor at propulsion system voltages (400- 800 V or more) might be preferable, they add, as the lower currents for the same power allow cables and connectors to be smaller and lighter. That also allows the compressor motor to reuse the available bus voltage, avoiding additional voltage conversions that reduce overall efficiency. However, higher voltages require more expensive power modules and more stringent isolation requirements, the control software and hardware expert notes. The HVAC, battery and charging systems expert says using the main battery voltage for the large power consumers is essential if the system is to be energy- and cost-efficient. He stresses that transferring 30% or more of battery capacity to a lower voltage will not only reduce efficiency but also produce high additional costs because of the need for the conversion hardware itself, for example. On the buses HVAC systems for large public service vehicles such as buses present their own efficiency challenges, the engineering simulation specialist Modern HVAC systems feature multiple sensors, actuators, power management, comms and control elements (Courtesy of STMicroelectronics) Autumn 2022 | E-Mobility Engineering 71 Focus | HVAC systems